1. Field of the Invention
The invention relates to transparentized paper and the process of making such paper.
2. Brief Description of the Prior Art
Transparent papers are widely used in the packaging, printing and engineering field. The purposes for using transparentized paper vary from reduced visual and reprint opacity to the chemical and physical resistance associated with them. Engineering and reproduction applications for transparentized papers require certain physical and chemical characteristics such as caliper ranges of between 0.001 and 0.015 inch, good pencil and ink receptivity for tracing media, combined with a resistance to uneven penetration of coating solutions used in reprographic papers. When these papers are used in drafting, visual brightness is required and high ultra-violet (UV) light translucency is necessary for making good reproductions. In practice, each grade of transparentized paper serves only a particular application. The commercial papers are generally not interchangeable with each particular application. It is also necessary that a transparentized paper have good physical strength to resist handling, good permanence for record keeping and chemical stability. Compositions applied to render the paper transparent should not migrate or volatilize (which would result in the loss of transparency and contamination of interfiled documents).
Paper is a fibrous material with many voids between fibers, which scatter incident light. The physical process of rendering paper transparent consists of filling the voids between the fibers with a material that has a refractive index identical or very close to that of the paper fibers. It should be noted that mineral fillers in papers affect the transparency because of their light absorption and light scattering properties and that papers without mineral fillers are preferred for transparentizing use.
Transparentizing paper is a widely established art and has been heretofore accomplished by impregnating the paper with mineral oils or waxes or polymeric thermoplastic resins, such as polystyrenes, polybutenes, polypropenes, various rosin derivatives, or various polymers or copolymers of acrylics and styrene, with or without plasticizers. The application of such materials to the paper is done by either hot melt applications or by applying organic solvent solutions or aqueous emulsions to the paper. The processing equipment used for applying or treating fibrous material such as paper, varies with the type of application employed. For example, the transparentizing mixture may be applied by extrusion, tub and roller applications. Excess material may be removed by a doctoring off process using scrapers, wire, squeeze rollers, or size press. Where paper is employed for transparentizing, it sometimes is "wet packed" after treatment to improve the distribution of the transparentizing agents in the body of the paper and then after a certain time the "wet packed" paper is passed through a dryer to remove the solvent vehicle. Where the transparentized paper is to be employed for drafting use, the transparentizing treatment is sometimes followed by a surface application with resins and toothing agents such as particles of silica which are harder than graphite.
There are many disadvantages and problems associated with the prior art processes for transparentized paper. High molecular weight resins would be desirable for transparentizing paper because of their physical strength, heat and water resistance, and absence of tendency of the resin to migrate within or out of the finished paper. Such resins, however, can be applied to the paper only with great difficulty since they tend to accumulate near or adjacent the surface of the paper and when impregnation is accomplished the resin is generally unevenly distributed throughout the paper. Thus, the translucency of the product made therefrom is low and is not uniform. Low molecular weight liquid resins of low viscosity or mineral oils readily penetrate the paper and produce a material of high translucency but the finished surface would be soft and would not be pencil resistant. The surface would "ghost" upon erasure. Also, low molecular weight transparentizing agents tend to migrate from the paper to interfiled documents lying adjacent thereto. Further, such low molecular weight resins or low viscosity materials are volatile and cannot be used where the transparentizing paper is subjected to elevated temperature processing equipment, such as xerography machines. The surface of such transparentized materials are often tacky to the touch and therefore are undesirable.
Polymeric transparentizing agents which contain polaric and non-polaric groups in the chain are wetting agents and will distribute more readily through the paper but form a transparentized paper which is too soft to the touch. Further, the transparentizing agent is mobile and easily leached out by water. It is not set in the paper and would not be suitable for the application, for example, of a diazo-sensitizing solution which is coated on the transparentized material. This is because such coating would deeply penetrate. The transparentizing process is generally carried out off the papermaking machine as a converting operation. Since the transparentizing materials generally contain organic solvents which are generally highly volatile aromatic liquids, the solvent vapors present pollution problems, not only to the atmosphere, but also in the area where the evaporation is taking place.
Many transparentized papers discolor when exposed to UV light and this affects the reprint speed.
Transparentized papers often produce excessive curl when coated with diazotype sensitizing solution. Such curl is objectionable when the treated paper is used.
Conventionally transparentized paper, probably due to the dielectric characteristics of the transparentizing resins, tend to accept and hold electrostatic charge during the handling and mechanical processing of the sheets with the result of collecting dust or jamming up automatic processing equipment.
A handling advantage of paper versus polymeric resin films lies in the fact that paper can be folded while film does not lend itself to folding. However, conventionally transparentized paper tends to leave pronounced crease marks when folded. Such crease marks are detrimental to the use of the transparentized paper for making diazo prints because the crease marks appear as dark lines on the print.
Diazotype copies on transparentized paper base often tend to cause print dye bleeding upon print aging. This means that the transparentizing agent in the paper exhibits solvent properties for the azo dye which upon aging moves slowly into the base and forms visible halos around the print lines. Recently, U.S. Pat. No. 3,813,261 disclosed transparentizing through impregnation of fibrous materials with partial ethers of polyols and methylol derivatives of polyamino compounds and thermal resin condensation in situ. This process while leading to highly transparentized sheets, has some shortcomings as the sheets become brittle in lower relative humidity atmospheres and tend to retain electrostatic charges.
U.S. Pat. No. 4,271,227.teaches transparentizing fibrous sheets through impregnation with ethylenically unsaturated ester monomers and catalytically initiated controlled thermal polymerization in situ. The process produces good translucency and water and solvent resistant sheets, which however tend to retain electrostatic charges and have lower folding resistance than the non-transparent sheets. Moreover, tinting dyes in the base are destroyed during the catalytic polymerization process.
In recent years acrylate ester impregnated photo polymerized transparentized paper has been offered which excels by its high degree of translucency. Such sheets however become brittle at low relative humidity, and leave extremely pronounced crease marks after folding. Moreover, they also tend to retain electrostatic charges.
It is an object of the present invention to overcome the disadvantages of heretofore conventionally transparentized paper by providing an improved transparentized paper with a high visual and UV light translucency, with improved tracing surfaces for good acceptance and erasability of pencil and ink lines, with an improved resistance against discoloration when exposed to UV light, with good receptivity of a diazo sensitizing solution and with high resistance to print dye bleeding.
Another object of the invention is to provide a transparentized paper of good performance which is resistant to water, and to common chemical solvents, and which is stable at elevated temperatures when being used in xerography machines.
Still another object of the invention is to provide a transparentized paper which is adapted for use in a variety of applications such as a base for diazo reproduction coatings, a tracing sheet, or a copying sheet in xerography machines.
A further object of the invention is to provide a process for transparentizing paper which can be performed without the need of a solvent and thereby avoids any pollution problems connected therewith.
A further object of the invention is to provide a transparentized paper useful as a diazotype intermediate which can be processed on conventional printing equipment without an objectional curl to the paper.
A further object of the invention is to provide a transparentized paper which does not tend to retain electrostatic charges in handling and processing.
Another object of the invention is to provide a transparentized paper which can be easily folded with a minimum degree of crease marks.
A further object of the invention is to provide a transparentized paper which is resistant to all conventional handling, but can be recycled for repulping by a simple aqueous alkaline pulping.
A further object of the invention is to provide a more economical transparentizing process for paper with readily available and low cost raw materials.